1. Field of the Invention
The present invention relates to single-component organopolysiloxane compositions prepared from hydroxylated diorganopolysiloxane oils, acid cross-linking agents comprising methyltriacetoxysilane and mixtures thereof with ethyltriacetoxysilane, and fillers, if appropriate. More especially, the present invention relates to such single-component organopolysiloxane compositions containing essential additives which retard or even completely eliminate the tendency of the acid cross-linking agents to crystallize.
2. Description of the Prior Art
The use of suitable means for preventing the crystallization of methyltriacetoxysilane at ambient temperature, or at lower temperatures, is known to this art; see French Patent No. 2,235,981, U.S. Pat. No. 4,116,935, French Patent No. 2,451,931 and published German Application No. 3,143,705.
French Patent No. 2,235,981 describes the use of an additive selective from the group consisting of dioxane, acrylonitrile, acetonitrile, trichloroethylene, butane sultone, butyl acetate and carbon tetrachloride to provide fungicidal activity and at the same time to depress the freezing point of methyltriacetoxysilane. The noted compounds, however, are either toxic or volatile (thus, half of these compounds have a flash point on the order of 40.degree. C. or lower); consequently, at the stage of production of the compositions it is already necessary to use an elaborate, and hence costly, apparatus to avoid any major accident; furthermore, at the point in time of use of such compositions it would be essential to work in a well-aired and ventilated environment and, in addition, when the incorporated additive is toxic, to avoid certain fields of applications, for example, those involving contact with foodstuffs or with pharmaceutical products.
U.S. Pat No. 4,116,985 describes the use of a mixture of methyltriacetoxysilane and 1,3-dimethyltetraacetoxydisiloxane containing at least 20% and not more than 80% of one of the two components. This mixture does not appear to crystallize at low temperature and it produces compositions which are easy to extrude.
However, the manufacture of this mixture is cumbersome, since it necessitates several heating stages and sparging with an inert gas; on an industrial scale, a colored product is usually obtained, which cannot be used for the manufacture of transparent silicone elastomers.
French Patent No. 2,451,931 describes the reaction of methyltriacetoxysilane or ethyltriacetoxysilane with tertbutanol and an aqueous solution of acetic acid; Examples 1 and 2 show that the products obtained do not crystallize at -10.degree. C. These products are complex mixtures which have the disadvantage of being difficult to reproduce; furthermore, they tend to form gels, which results in compositions which are heterogeneous in structure in thin layers.
German Application No. 3,143,705 teaches the use of formic acid to depress the crystallization temperature of methyltriacetoxysilane; this additive is claimed to be more effective than acetic acid or dioxane. It is well known, however, that formic acid can be used only with great care because of its corrosive nature in respect of many substances and of its stinging effect on skin; the compositions in which it is present can therefore be used only in fairly restricted fields of applications.